1. Field of the Invention
The present invention relates in general to a method of joining together two planar members by friction stir welding, and a tab plate used in the method, and more particularly to such a method that permits sound welding at the end portion of a weld region at which the friction stir welding operation is terminated, and a tab plate suitably used in the friction stir welding operation.
2. Discussion of Related Art
Recently, an increasing attention has been drawn to a friction stir welding process, as one of welding processes which are used for joining together two planar members and which involve a relatively small amount of heat generation, a relatively small amount of strength reduction in the weld region and a relatively small amount of residual strain in the weld region. Examples of the friction stir welding process are disclosed in U.S. Pat. No. 5,460,317 and JP-A-7-505090.
Described in detail, the friction stir welding process is performed by using a rotary tool or wheel 3, which has a cylindrical body 6 in the form of a round rod having a circular shoulder surface 5 at its bottom end, and a probe 4 in the form of a pin formed on the shoulder surface 5 such that the probe 4 is coaxial with the shoulder surface 5, as shown in FIG. 1. The probe 4 is formed of a harder material than the materials of two planar members 2a, 2b to be joined together by a friction stir welding operation. In the friction stir welding operation, the rotary tool 3 rotated at a high speed is moved relative to a joint region 8 defined by end portions of the two planar members 2a, 2b which are butted together. The rotary tool 3 is moved relative to the joint region 8 such that the shoulder surface 5 is held in rubbing contact with the surface of the joint region 8 while the probe 4 is held inserted in the joint region 8, so that frictional heat is generated between the rotating probe 4 and shoulder surface 5 and the planar members 2a, 2b, whereby the material at the joint region 8 and in the vicinity of the joint region 8 is plasticised by the generated frictional heat, while the plasticised segments of the butted end portions of the two planar members 2a, 2b are interlocked with each other, as a result of a stirring action of the probe 4 rotating at the high speed. Thus, the two planar members 2a, 2b are jointed together without melting of their materials.
The friction stir welding operation described above is terminated at one of opposite longitudinal end portions of the joint region 8 of the planar members 2b, 2a. Described more precisely, the rotary tool 3 is lifted to remove the probe 4 from the joint region 8, at a position which is spaced by at least 10 mm away from the end faces of the planar members 2a, 2b corresponding to the above-indicated one longitudinal end portion of the joint region 8, in the direction toward the other longitudinal end portion. The friction stir welding operation is terminated at the above-indicated position of the joint region 8, in order to prevent occurrence of a joining or welding defect at the above-indicated one longitudinal end portion of the joint region 8, which would be caused by an escape flow of the material plasticised and stirred by the rotating rotary tool 3, from the corresponding end of the joint region 8. As a result, a weld region 10 formed by the friction stir welding operation performed on the joint region 8 does not reach the end faces of the two planar members 2a, 2b on the side of termination of the friction stir welding operation, so that a non-weld region 12 remains at the above-indicated longitudinal end portion of the joint region 8, as shown in FIG. 1, over a certain length not smaller than 10 mm between the corresponding end of the joint region 8 (end faces of the planar members 2a, 2b) and the periphery of the circular shoulder surface 5 (the corresponding end of the formed weld region 10) upon termination of the friction stir welding operation. Accordingly, it is necessary to rectify an assembly of the two planar members 2a, 2b joined together, by removing the non-weld region 12 or performing a supplemental fusion welding operation at the non-weld region 12. This rectifying operation performed after the friction stir welding operation not only reduces the efficiency of the overall welding operation, but also causes some other drawbacks such as residual strain and softening of the material due to the supplemental fusion welding operation, and formation of a cavity or recess at the end of the weld region 10 due to removal or separation of the probe 4 and shoulder surface 5 of the rotary tool 3 away from the joint region 8.
To solve such drawbacks that would be caused by the termination of the friction stir welding operation, there has been proposed a friction stir welding technique wherein each of the two workpieces to be joined together is provided with a dummy end section corresponding to the end portion of the joint region at which the friction stir welding operation is terminated. After the friction stir welding operation, the dummy end sections of the two workpieces are cut off, so that an obtained assembly of the workpieces joined together does not have a cavity or recess which would be left due to the removal or separation of the probe 4 and shoulder surface 5 of the rotary tool 3. An example of this technique is disclosed in JP-A-2000-42759. However, this technique still has some problems such as a need of forming the workpieces with the dummy end sections, and an inevitably increased cost of manufacture of the desired assembly due to the removal of some amount of material from the workpieces, and an operation to remove the dummy sections.
The friction stir welding process is also performed to join together two tubular members such that an abutment jig is in abutting contact with a joint region of the two tubular members, and such that the friction stir welding operation is terminated by moving the rotary tool from the terminal end of the joint region onto the abutting jig, for thereby preventing undesirable formation of a cavity or recess due to the removal of the rotary tool from the end portion of the joint region. An example of this technique is disclosed in JP-A-2000-42762. However, this technique suffers from a problem of difficulty to remove the abutting jig from the weld region in the presence of a stirred metal material at the interface between the weld region and the abutting jig. An excessively large force if applied to the interface to remove the abutting jig would cause a part of the abutting jig to remain at the interface, resulting in a need of rectifying an obtained assembly of the two tubular members, or giving rise to a risk of occurrence of a joining or welding defect due to a partial removal of the weld region. Thus, this technique is not desirable to assure a sound weld region in the obtained assembly of the tubular members.